If your vimrc is configured to use backup files and you try to invoke crontab -e (or vipw or visudo, etc.), you might get an error like this:
crontab: temp file must be edited in place
You need to make vim not do backups of temp files. An easy way to fix this is to add something like this to your vimrc file:
set backupskip=/tmp/*,/private/tmp/*